Digital magnetic storage cell device

ABSTRACT

Digital magnetic memory cell device for read and/or write operations, having a soft-magnetic read and/or write layer system and at least one hard-magnetic reference layer system formed as an AAF system, comprising an AAF layer composite and at least one reference layer, the reference layer system comprising at least one ferromagnetic layer arranged adjacent to a magnetic layer of the AAF layer composite, in which case the thickness of the antiferromagnetic layer ( 10 ) is dimensioned in such a way that it has a uniaxial anisotropy.

[0001] The invention relates to a digital magnetic memory cell device for read and/or write operations, having a soft-magnetic read and/or write layer system and at least one hard-magnetic reference layer system formed as an AAF system, comprising an AAF layer composite and at least one reference layer, the reference layer system comprising at least one antiferromagnetic layer arranged adjacent to a magnetic layer of the AAF layer composite.

[0002] A digital memory cell device of this type serves for storing information on a magnetic basis. An individual memory cell device is generally part of a memory device, often also called MRAM (magnetic random access memory). A memory of this type can be used to carry out read and/or write operations. Each individual memory cell device comprises a soft-magnetic read and/or write layer system, which is separated by means of an intermediate layer from a hard-magnetic reference layer system, formed as an AAF system in the case of the present type of memory cell device. The magnetization of the reference layer of the reference layer system is stable and does not change in an applied field, while the magnetization of the soft-magnetic read and/or write layer system can be switched by means of an applied field. The two magnetic layer systems can be magnetized parallel or antiparallel with respect to one another. The two aforementioned states in each case represent a bit of information, i.e. the logic zero (“0”) state or one (“1”) state. If the relative orientation of the magnetization of the two layers changes from parallel to antiparallel, or vice versa, then the magnetoresistance changes over this layer structure by a few percent. This change in resistance can be used for the read-out of digital information stored in the memory cell. The change in the cell resistance can be identified by a voltage change. By way of example, the cell may be occupied by a logic zero (“0”) in the event of a voltage increase and the cell may be occupied by a logic one (“1”) in the event of a voltage decrease. Particularly large resistance changes in the region of a few percent have been observed when the magnetization orientation changes from parallel to antiparallel and vice versa in cell structures of the GMR type (giant magnetoresistance) or the TMR type (tunnel magnetoresistance).

[0003] An important advantage of such magnetic memory cells is that the information is stored in a persistent manner in this way, and is stored without maintenance of any basic supply even with the device switched off and is immediately available again after the device is switched on, unlike in known conventional semiconductor memories.

[0004] A central constituent part in this case is the reference layer system formed as an AAF system (AAF=artificial antiferromagnetic). Such an AAF system is advantageous on account of its high magnetic rigidity and the relatively low coupling to the read and/or write layer system through the so-called orange peel effect and/or through macroscopic magnetostatic coupling fields. An AAF system generally comprises a first magnetic layer or a magnetic layer system, an antiferromagnetic coupling layer and a second magnetic layer or a magnetic layer system which is coupled by its magnetization, via the antiferromagnetic coupling layer, oppositely to the magnetization of the lower magnetic layer. Such an AAF system can be formed e.g. from two magnetic Co layers and an antiferromagnetic coupling layer made of Cu.

[0005] In order to improve the rigidity of the AAF system, that is to say its resistance to external outer fields, it is customary to arrange an antiferromagnetic layer at the magnetic layer of the AAF system which is remote from the read and/or write layer system. By means of this antiferromagnetic layer, the directly adjacent magnetic layer is additionally pinned in its magnetization, so that overall the AAF system becomes harder (exchange pinning or exchange biasing).

[0006] The magnetic rigidity of the AAF system corresponds to the amplitude of the applied external field which is required for rotating the magnetizations of the two ferromagnetic layers in the same direction, that is to say for parallel setting. This limits the magnetic window for read and write applications of such a memory cell device.

[0007] The invention is based on the problem of specifying a memory cell device which has a larger magnetic window or operation window.

[0008] In order to solve this problem, in the case of a memory cell device of the type mentioned in the introduction, it is provided according to the invention that the thickness of the antiferromagnetic layer is dimensioned in such a way that it has a uniaxial anisotropy.

[0009] In known memory cell devices, the antiferromagnetic layer, which pins the magnetization of the ferromagnetic layers situated above it by exchange biasing, has a considerable thickness, which results in a unidirectional anisotropy. However, a unidirectional anisotropy leads to asymmetrical hysteresis curves which, for their part, in turn lead to narrower magnetic windows or operation windows.

[0010] The invention is now based on the insight that a hysteresis curve that is symmetrical to the greatest possible extent can be achieved when the antiferromagnetic layer is so thin that it has only a uniaxial anisotropy. This makes it possible to significantly widen the magnetic window with the use of the same materials for forming the memory cell device. Furthermore, the thermal stability of the device is increased, since the blocking temperature, that is to say the temperature above which the anisotropy of the antiferromagnetic layer is lost, increases, which is advantageous with regard to the thermal stability of the entire system.

[0011] In this case, it is expedient if the antiferromagnetic layer is arranged at that side of the AAF layer composite which is remote from the read and/or write layer system. The thickness of the antiferromagnetic layer is expediently chosen in a manner dependent on the layer material used and is always ≦10 nm. The antiferromagnetic layer may comprise any desired material used in known fashion to construct such memory cell devices, e.g. NiO, FeMn, IrMn, NiMn, PtMn, CrPtMn, RhMn or PdMn.

[0012] The memory cell device itself may be a giant magnetoresistive, a magnetic tunnel junction or a spin valve transistor device.

[0013] In addition to the memory cell device itself, the invention furthermore relates to a digital memory device comprising a plurality of memory cell devices of the type described.

[0014] Further advantages, features and details of the invention emerge from the exemplary embodiment described below and also with reference to the drawings, in which:

[0015]FIG. 1 shows a basic diagrammatic sketch of a memory cell device according to the invention, and

[0016]FIG. 2 shows a diagram for illustrating the hysteresis curves resulting in the case of a uniaxial and a unidirectional anisotropy.

[0017]FIG. 1 shows a memory cell device 1 according to the invention in a first embodiment. This comprises a reference layer system 2, which is decoupled from a soft-magnetic read and/or write layer system 4 via a decoupling layer 3. The illustration furthermore shows the word and bit lines 5 a, 5 b running above and below at right angles to one another. The reference layer system 2 itself is arranged on a substrate 5. The reference layer system 2 comprises an AAF layer composite 6, comprising a lower ferromagnetic layer 7, an upper ferromagnetic layer 8 and a coupling layer 9 which is arranged between said layers and couples in antiparallel fashion. The ferromagnetic layers may be made e.g. of Co and the coupling layer which couples in antiparallel fashion may be made of Cu. The construction of such an AAF layer composite is adequately known.

[0018] The reference layer system 2 furthermore comprises an antiferromagnetic layer 10 which is provided below the lower ferromagnetic layer 7 and may be made e.g. of NiO, FeMn, IrMn, NiMn, PtMn, CrPtMn, RhMn or PdMn. The antiferromagnetic layer 10 couples the magnetization of the lower ferromagnetic layer 7 situated above it, i.e. the latter is oriented parallel to the magnetic moments of the antiferromagnetic layer in the interface region. As a result of this, the magnetization of the ferromagnetic layer 7 is pinned by exchanged biasing.

[0019] The thickness of the antiferromagnetic layer 10 is now dimensioned in such a way, or the layer is deposited so thin, that it has only a uniaxial anisotropy. The layer thicknesses are significantly smaller than the typical thickness values for antiferromagnetic coupling layers which couple through exchange biasing. By way of example, in the case of an antiferromagnetic layer made of IrMn, the customary layer thickness is approximately 8-10 nm. With such a thick layer, however, a unidirectional anisotropy is established which adversely affects the width of the magnetic window or the hysteresis curve and the thermal stability of the memory cell device.

[0020] The layer thickness—according to the invention—of the antiferromagnetic layer 10 is significantly smaller; in the case of the exemplary embodiment of the layer made of IrMn, e.g. merely 2-3 nm. This has the effect that a uniaxial rather than a unidirectional anisotropy is established. This makes it possible, as shown clearly by FIG. 2, to obtain a symmetrical hysteresis curve which exhibits a significantly wider magnetic window. In FIG. 2, the solid curve I shows the profile of the external field which is applied to the memory cell device in order to rotate the magnetizations, in the case of a very thin antiferromagnetic layer 10 having only a uniaxial anisotropy. The curve I is symmetrical and the magnetic window is considerably wide. By contrast, the dashed curve II shows an example of the profile of the magnetization curve in the case of a unidirectional anisotropy. The curve is clearly asymmetrical and the magnetic window is significantly narrower.

[0021] In addition to an improvement with regard to widening of the magnetic window, the use of a very thin antiferromagnetic layer 10 is also advantageous to the effect of increasing the thermal stability of the device. The blocking temperature, that is to say the temperature above which the anisotropy of the layer is lost, of an antiferromagnetic layer made of IrMn can be increased from approximately 250° C. in the case of relatively thick layers within the above-specified larger thickness range to as much as approximately 400° C. in the case of significantly thinner layers.

[0022] List of reference symbols

[0023]1 Memory cell device

[0024]2 Reference layer system

[0025]3 Decoupling layer

[0026]4 Read and/or write layer system

[0027]5 a Word and bit lines

[0028]5 b Word and bit lines

[0029]6 AAF layer composite

[0030]7 Lower ferromagnetic layer

[0031]8 Upper ferromagnetic layer

[0032]9 Coupling layer

[0033]10 Antiferromagnetic layer 

1. Digital magnetic memory cell device (1) for read and/or write operations, having a soft-magnetic read and/or write layer system (4) and at least one hard-magnetic reference layer system (2) formed as an AAF system, comprising an AAF layer composite (6) and at least one reference layer, the reference layer system (2) comprising at least one antiferromagnetic layer (10) arranged adjacent to a magnetic layer (7) of the AAF layer composite (6), characterized in that the thickness of the antiferromagnetic layer (10) is dimensioned in such a way that the antiferromagnetic layer (10) has a uniaxial anisotropy.
 2. Memory cell device according to claim 1, characterized in that the antiferromagnetic layer (10) is arranged at that side of the AAF layer composite (6) which is remote from the read and/or write layer system (4).
 3. Memory cell device according to claim 1 or 2, characterized in that the thickness of the antiferromagnetic layer (10) is chosen in a manner dependent on the layer material used and is always ≦10 nm.
 4. Memory cell device according to one of claims 1 to 3, characterized in that the antiferromagnetic layer (10) is chosen from a material from the group NiO, FeMn, IrMn, NiMn, PtMn, CrPtMn, RhMn and PdMn.
 5. Memory cell device according to one of claims 1 to 4, characterized in that it is a giant magnetoresistive, a magnetic tunnel junction or a spin valve transistor system.
 6. Digital memory device, comprising a plurality of memory cell devices according to one of claims 1 to
 5. 